An exemplary traveling wave tube (TWT) 20 is illustrated in FIG. 1. The elements of TWT 20 are generally coaxially-arranged along a TWT axis 22. The elements include an electron gun 24, a slow wave structure 26 (embodiments of which are shown in FIGS. 2A and 2B), a beam focusing arrangement 28 which surrounds slow wave structure 26, a microwave signal input port 30 and a microwave signal output port 32 which are coupled to opposite ends of slow wave structure 26, and a collector 34. A housing 36 is typically provided to protect the TWT elements.
In operation, electron gun 24 injects a beam of electrons into slow wave structure 26. The electron beam has a given power level. Beam focusing arrangement 28 guides the electron beam through slow wave structure 26. A microwave input signal 38 is inserted at input port 30 and moves along slow wave structure 26 to output port 32. Slow wave structure 26 causes the phase velocity (i.e., the axial velocity of the phase front of the signal) of the microwave signal to approximate the velocity of the electron beam.
As a result, the electrons of the beam are velocity modulated into bunches which interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal causing the signal to be amplified. The amplified signal is coupled from output port 32 as a microwave output signal 40. After their passage through slow wave structure 26, the electrons are collected in collector 34.
The amount of kinetic energy transferred from the electrons to the microwave signal is approximately constant at low microwave signal input power levels. Thus, the gain between the microwave output and input signals is constant. As the power of the microwave signal input increases, nonlinear effects become more significant. Eventually, the microwave output signal reaches a maximum power value and the TWT operates at saturation.
Approaching saturation, the relationship between the microwave output and input signals becomes nonlinear. If the power of the microwave input signal is increased further beyond saturation, the power of the microwave output signal and the gain decrease. A TWT operating below its saturated microwave output power level is referred to as running "backed off" from saturation. The amount of "back off" is the difference in dB between the power levels of "backed off" and saturated microwave output signals. A TWT running at least 3 dB "backed off" from saturation provides a very high amplitude and phase linearity needed for communication applications.
Beam focusing arrangement 28 is configured to develop a magnetic field for guiding the electron beam through slow wave structure 26. A first configuration includes a series of annular, coaxially arranged permanent magnets 42 which are separated by pole pieces 44. Magnets 42 are arranged so that adjacent magnet faces have the same magnetic polarity. This beam focusing configuration is comparatively light weight and is referred to as a periodic permanent magnet (PPM) arrangement. In TWTs in which output power is more important than size and weight, a second configuration often replaces the PPM with a solenoid 46 (partially shown adjacent input port 30) which carries a current supplied by a solenoid power supply (not shown).
As shown in FIGS. 2A and 2B, TWT slow wave structures generally receive an electron beam 48 from electron gun 24 (see FIG. 1) into an axially repetitive structure. A first exemplary slow wave structure is helix member 50 shown in FIG. 2A. A second exemplary slow wave structure is coupled cavity circuit 52 shown in FIG. 2B. Coupled cavity circuit 52 includes annular webs 54 which are axially spaced to form cavities 56. Each of annular webs 54 form a coupling hole 58 which couples a pair of adjacent cavities. Helix member 50 is especially suited for broad band applications while coupled cavity circuit 52 is especially suited for high power applications.
Electron gun 24, helix member 50, and collector 34 are again shown in the TWT schematic of FIG. 3. Electron gun 24 has a cathode 60 and an anode 62. Collector 34 has a first annular collector stage 64, a second annular collector stage 66, and a third collector stage 68. Because third collector stage 68 generally has a cup-like or bucket-like form, it is sometimes referred to as the "bucket" or "bucket stage".
Helix member 50 and a body 70 of TWT 20 are at ground potential. Cathode 60 is biased negatively by a voltage V.sub.cath from a cathode power supply 72. An anode power supply 74 is referenced to cathode 60 and applies a positive voltage to anode 62. This positive voltage establishes an acceleration region 76 between cathode 60 and anode 62. Electrons are emitted by cathode 60 and accelerated across the acceleration region 76 to form electron beam 48. The positive and negative terminals of the anode power supply 74 and the cathode power supply 72 are indicated in FIG. 3 by (+/-), respectively.
As described above with reference to FIG. 1, electron beam 48 travels through helix member 50 and exchanges energy with a microwave signal which travels along the helix member from input port 30 to output port 32. Only a portion of the kinetic energy of electron beam 48 is transferred in the energy exchange. Most of the kinetic energy remains in electron beam 48 as it enters collector 34. The electron beam entering collector 34 is referred to as the spent electron beam. A significant part of the kinetic energy of the spent electron beam can be recovered by decelerating the electrons before they are collected by the collector stages.
Because of their negative charge, the electrons of electron beam 48 form a negative "space charge" which causes the electron beam to radially disperse in the absence of any external restraint. Accordingly, beam focusing arrangement 28 applies a magnetic field which restrains the radial divergence of the electrons by causing them to spiral about the beam.
However, electron beam 48 is no longer under this restraint when it enters collector 34 and, consequently, it begins to radially disperse. In addition, the interaction between electron beam 48 and the microwave signal on slow wave structure 26 causes the electrons to have a "velocity spread" as they enter collector 34, i.e., the electrons have a range of velocities and kinetic energies. Depending upon the amount of interaction, some of the electrons may have radial as well as axial velocity components. In short, the microwave signal perturbs electron beam 48. The degree of perturbance is much larger at saturation than at backed off operation.
Negative voltages are applied to collector 34 to achieve electron deceleration. The potential of collector 34 is "depressed" from that of TWT body 70 (i.e., made negative relative to the TWT body). The kinetic energy recovery is further enhanced by using a multistage collector, e.g., collector 34, in which each successive stage is further depressed from the body potential of V.sub.B. For example, if first collector stage 64 has a potential V.sub.1, second collector stage 66 has a potential V.sub.2, and third collector stage 68 has a potential of V.sub.3, these potentials are typically related by the equation V.sub.B =0&gt;V.sub.1 &gt;V.sub.2 &gt;V.sub.3 as indicated in FIG. 3. The efficiency of the collector in collecting the kinetic energy from the spent electron beam is referred to as the collection efficiency.
The voltage V.sub.1 on first collector stage 64 is depressed sufficiently to decelerate the slowest electrons 80 in electron beam 48 and yet still collect them. If this voltage V.sub.1 is depressed too far, first stage 64 repels rather than collects electrons 80. These repelled electrons may flow to TWT body 70 and reduce the efficiency of TWT 20. Alternatively, they may reenter the energy exchange area of helix member 50 and reduce the stability of TWT 20.
Similar to first collector stage 64, successively depressed voltages are applied to successive collector stages to decelerate (but still collect) successively faster electrons in electron beam 48, e.g., electrons 82 are collected by second collector stage 66 and electrons 84 are collected by third collector stage 68.
In operation, the diverging low kinetic energy electrons 80 are repelled by second collector stage 66, which causes their divergent path to be modified so that they are collected on the interior face of the less depressed collector stage 64. Higher energy electrons 82 are repelled by collector stage 68, which causes their divergent paths to be modified so that they are collected on the interior face of the less depressed collector stage 66. Finally, the highest energy electrons 84 are decelerated and collected by collector stage 68. This process of improving the efficiency of TWT 20 by decelerating and collecting successively faster electrons with successively greater depression on successive collector stages is generally referred to as "velocity sorting".
To recover a large fraction of the power of the spent electron beam, the stages must be designed to sort the electrons in the spent beam into various energy classes. Then, electrons in each energy class must be collected on a collector stage at a voltage that recovers as much of that energy as possible.
The gain in the collection efficiency realized by velocity sorting of electron beam 48 can be further understood with reference to current flows through a collector power supply 86 which is coupled between cathode 60 and collector stages 64, 66, and 68. The positive and negative terminals of the collector power supply 86 are indicated on FIG. 3 by (+/-), respectively. If the potential of collector 34 were the same as TWT body 70, the total collector electron current I.sub.coll would flow back to cathode power supply 72 as indicated by current 88 in FIG. 3, and the input power to TWT 20 would substantially be the product of the cathode voltage V.sub.cath and the collector current I.sub.coll.
In contrast, the currents of collector 34 flow through collector power supply 86. The input power associated with each collector stage is the product of that stage's current and its associated voltage in collector power supply 86. Because the voltages V.sub.1, V.sub.2, and V.sub.3 of collector power supply 86 are a fraction (e.g., in the range of 30-70%) of the voltage of cathode power supply 72, the TWT input power is effectively decreased.
To increase the collection efficiency, it is desirable that as much of the electron beam as possible is collected by the most negatively depressed stages. It is also desirable that the voltages of the most negatively depressed stages are as large a fraction of the voltage of cathode power supply 72 as possible. It is further desirable that many collector stages be employed in the collector such that many different voltages corresponding to the electron energy classes are applied to the stages.
Efficiencies of TWTs with multistage collectors are typically in the range of 25-60%, with higher efficiency generally associated with narrower bandwidth. These efficiencies can be further improved by enhancing the velocity sorting of collector 34 and considerable efforts have been expended towards this goal in the areas of collector design, simulation, and prototype testing.
However, a problem with successively depressing collector stages to gradually decelerate an electron beam to recover kinetic energy is that this causes high perveance and/or significantly perturbed electron beams to diverge rapidly. Perveance is a measure of the electron beam space charge. Rapid divergence physically limits the ability of the electron beam to reach the most highly depressed collector stages thereby limiting the collector efficiency.